The invention relates to a coolable blade for a gas turbine, or the like, having a blade body and a blade root.
Such a blade has been disclosed, for example, by German Patent Application 198 60 788.1, on which the invention is based. It essentially comprises a blade body and a blade root with an integrated cooling system. The blade body is composed of a suction-side wall and a pressure-side wall, which, while forming a cavity, are connected via a leading edge, a trailing edge and a blade tip. The walls define the profile shape and enclose the cavity within, which is utilized for cooling purposes. To this end, the cavity is subdivided into passages by essentially radially running separating webs. In the triple-pass cooling system described in this publication, a first separating web, starting from the blade root, runs radially outward right into the vicinity of the blade tip, and a second separating web, starting from the blade tip, runs right into the vicinity of the blade-root region. In this way, the cavity is subdivided into three radially running passages, which form a continuous flow path which is deflected twice and through which a cooling medium can flow. As a rule, the cooling medium is fed through the blade root, for example in a plane corresponding to the leading edge. When flow occurs through the passages, the cooling medium absorbs heat introduced from outside and leaves the blade in the region of the trailing edge. To this end, a row of blow-out openings, for example, are provided; there may also be a blow-out slot which is largely continuous in the radial direction.
Although such a cooling concept has proved successful in principle, problems have partly occurred, and these problems necessitate further improvements.
A first problem area lies in the fact that the cooling medium experiences a deflection of about 180xc2x0 at the transition from one passage to the neighboring passage, as a result of which there is a risk of flow separation. Such a flow separation is undesirable, since there is an increased flow loss in the region concerned and the rate of flow of cooling medium is reduced. Furthermore, such flow-separation zones are extremely unstable, so that no constant throughput of cooling medium can be produced. As a result, local or even complete overheating of the blade may be observed, which in the most unfavorable case leads to total loss.
To avoid such flow-separation zones, baffle plates, for example, are in use; attempts are also made to deliberately influence the passage geometry by local contouring (thickening) of the separating webs. However, this leads to an increased cost in terms of design or production.
The region of the blade tip, in particular in the vicinity of the blade trailing edge, poses a further problem. This region corresponds with the rear passage (as viewed in the direction of flow), through which a cooling medium which is already greatly heated flows. The temperature gradient toward the wall and available for the heat transfer has greatly decreased relative to the inlet region. In addition, the quantity of cooling medium available for the heat dissipation is already greatly reduced as a result of the cooling air blown out via the trailing edge, so that sufficient cooling is exceptionally problematic overall, especially in the region of the blade tip at the trailing edge.
The object of the invention, in attempting to avoid the disadvantages described above, is to specify a coolable blade for a gas turbine, or the like, having a blade body and a blade root, in which, it is possible to increase the cooling effect in the region of the blade tip and/or the trailing edge and thereby prolong the service life thereof.
According to the invention, this is achieved in that, in a coolable blade having a blade body and a blade root, the separating web which is adjacent to the trailing edge is provided with at least one through passage close to the blade tip. The result of this is that some of the cooling medium, during the deflection, is branched off from the passage assigned to the leading edge into the center passage and is fed directly to the third passage assigned to the trailing edge. Thus, cooling medium, which has a comparatively low temperature, passes into the region which is especially at risk of overheating. Since a lower pressure prevails in the trailing-edge regionxe2x80x94and thus in the rear passagexe2x80x94than in the center passage, the cooling medium is drawn off at high velocity through the through passage. This effect also helps considerably to improve the cooling effect.
In addition, the drawing-off of cooling medium through the through passage prevents a flow separation in this region as a result of the deflection during the transfer from the front passage to the center passage. The thickening of the separating web practiced hitherto in this region may be dispensed with.
Taking this basic concept as the starting point, specific adjustment of the bypass of the cooling-air transfer may be carried out by the variants described below in such a way that the requisite heat dissipation is exactly achieved.
To optimize the flow conditions in the region referred to, not only the number and arrangement of the through passages but also the cross-sectional profile of the through passages may be varied. To set a predetermined entry velocity of the cooling medium into the rear passage, the cross section may be designed to diverge or converge in the direction of flow.
For a number of applications, it has proved to be expedient to assign a guide web to the through passage or through passages, as a result of which an essentially axially running tip cooling passage is obtained. This configuration is of particular importance especially at particular high thermal loads in this region, as may be observed in the case of blades with a free end and blades with a crown. Depending on the requirement, the guide web may be arranged so as to be continuous between the separating web and the trailing edge, so that mixing of the drawn-off partial flow with the deflected main flow is completely prevented. Alternatively, the guide web may be provided with essentially radially running through-holes or else be composed of individual segments arranged at a distance from one another, so that partial mixing of both partial cooling flows is permitted.
Furthermore, the guide web may have a curved contour section, so that a flow separation of the cooling medium after entry into the rear passage is avoided. The guide web thus has the function of a baffle plate or a deflecting rib. Furthermore, discharge passages may be arranged in the region of the blade tip in order to specifically assist locally the cooling of the blade.
Finally, in the rear passage and/or in the tip passage in the region of the blade tip and/or the trailing edge, additional cooling elements may be provided on the inside of the wall or so as to be continuous between the suction-side wall and the pressure-side wall. Such cooling elements are built-in components which enlarge the surface required for the heat transfer and intensify the heat transfer. Especially effective are cooling elements in the form of semi-cylinders, spherical sections, ribs or cylinders.